The present invention relates to microcapsules comprising a core of a liquid substance and a wall of an inorganic substance and a process for the preparation thereof. More particularly, the present invention relates to microcapsules of excellent properties having a wall of a highly chemically stable, highly heat-resistant inorganic substance, and capable of preventing the liquid core substance for oozing out from the capsules with the lapse of time.
Conventional process for preparing microcapsules comprising a core of a liquid substance and a outer shell wall of an inorganic substance can be classified roughly into a process wherein precipitation reaction is employed and a process wherein adhesion or forced adhesion of the inorganic wall substance is employed. However, it is difficult to form dense, uniform outer shell walls of capsules by any of those processes. In other words, since the outer wall of the capsule has very small cracks, openings or pin holes and, in addition, the wall is coarse or rough, the liquid core substance enclosed in the capsule is apt to be modified under the influence of the atmosphere such as oxygen, carbon dioxide and water contained in air, or apt to absorb water in air. Thus, the capsule has a defect that the core substance cannot be protected sufficiently stably. Further, if the core substance enclosed in the capsule is a volatile substance, the core substance is vaporized through the outer shell wall of the capsule. If the core substance has a strong smell, the smell is emitted through the outer shell wall of the capsule. if a perfume is used as the core substance, it is hard to protect the same for a long period of time due to vaporization thereof unfavorably.
Thus, according to the known processes for preparing microcapsules having an inorganic wall substance, it is difficult to protect a liquid core substance stably over a long period of time, since not only a chemically unstable substance, volatile substance and hygroscopic substance but also other substances may ooze out from the capsule through small openings of the outer shell wall or these liquid core substances may be influenced by, for example, oxidation by air. Those prior art processes have, therefore, been employed in very limited fields.
In another known process, a liquid substance is charged into a previously formed microballoon comprising a wall of inorganic substance. Such a capsule is prepared by treating the hollow microballoon under a high pressure to form cracks therein and then allowing the liquid substance to be absorbed in the balloon through the cracks. Therefore, the liquid core substance vaporizes or leaks out through the cracks formed in the wall to make it impossible to obtain an ever-stable capsule. Further, there has been attempted a process wherein the wall of inorganic substance is further coated with an organic high polymer substance to prevent the vaporization or leakage of the core substance. However, this coating process inevitably has a defect that superior properties of the micro-balloon, i.e. chemical stability and heat resistance due to the inorganic wall substance, cannot be exhibited sufficiently.